1. Field of the Invention
The present invention pertains, in general, to dielectric compositions, and in particular, to a dielectric composition having increased homogeneity and insulation resistance, comprising a main ingredient and a supplementary ingredient calcined while controlling their particle size in combination with a separately added sintering aid; a method of preparing the same and a multi-layer ceramic capacitor using the same.
2. Description of the Prior Art
Generally, as high dielectric constant materials showing Y5V temperature property, use has been made of a mixture of (Ba1xe2x88x92xCax)m(Ti1xe2x88x92yZry)O3+m prepared by synthesizing BaCO3, CaCO3, TiO2 and ZrO2 as a principal ingredient through solid phase reaction, and various additives as a supplementary ingredient for improving electrical properties.
In this regard, the main ingredient powder composed of barium titanate (BaTiO3), barium zirconate (BaZrO3), calcium titanate (CaTiO3) and calcium zirconate (CaZrO3) sintered together with various auxiliary ingredient powders is used as a dielectric layer in a multi-layer ceramic capacitor with an internal Ni electrode meeting Y5V property. As for additives, used are manganese oxides, acting as an acceptor for providing resistance to reduction by oxygen vacancy formed previously within a master material, at least one selected from among compounds of rare-earth elements, such as Y2O3, Ho2O3, Er2O3, Dy2O3 and V2O5, acting as a donor for supplementing electric charges used for facilitated deterioration of insulation resistance by thusly formed oxygen vacancy, and a sintering aid of a glass component.
The main ingredient (Ba1xe2x88x92xCax)m(Ti1xe2x88x92yZry)O3+m powder, which is prepared by solid phase synthesis, can be regulated in its particle size by controlling temperature and reaction time of a thermal treatment, and thus is advantageous in light of relatively simple preparation process and low preparation cost.
However, since a certain additive is simply added to (Ba1xe2x88x92xCax)m(Ti1xe2x88x92yZry)O3+m in a batch process, homogeneity of the mixture comprising the main ingredient and the supplementary ingredient is reduced, and thus a microstructure of the dielectric composition becomes nonhomogeneous.
When (Ba1xe2x88x92xCax)m(Ti1xe2x88x92yZry)O3+m is not uniformly mixed with the additive and is locally segregated, such segregation adversely affects particle growth and thus a nonhomogeneous microstructure distribution is obtained.
Such nonhomogeneity causes variation of electrical properties as well as severe problems related to reliability.
As for mixing of the additive, it is thus important that (Ba1xe2x88x92xCax)m(Ti1xe2x88x92yZry)O3+m is uniformly mixed with the additive.
Particularly, according to recent tendency toward miniaturization and thinness of multi-layer ceramic capacitors, the dielectric layer becomes thin, whereby a uniform mix of (Ba1xe2x88x92xCax)m(Ti1xe2x88x92yZry)O3+m and the additive is a chief process variable for preparation of such capacitors having high capacities.
Typically, in the case of merely adding previously calcined main ingredient powders with and a small amount of an additive in a batch process, the extent of dispersion cannot be controlled. As such, the additive is responsible for controlling electrical properties, such as provision of resistance to reduction, deterioration prevention of insulation resistance and improved reliability in the multi-layer ceramic capacitor. Upon application of such a dielectric composition to the ceramic capacitor having high capacity, nonhomogeneous microstructure results in irregular voltage resistance and poor reliability.
In order to solve such problems, there have been proposed mixing methods over long periods of time to improve dispersibility of raw material powders. But such methods suffer from disadvantages of increased process cost due to long treatment time, and adulteration by impurities.
Japanese Laid-open Pat. No. 2000-243652 discloses a dielectric composition prepared by simultaneously calcining a main ingredient, a supplementary ingredient and a sintering aid. However, the above patent has the drawback of large particle size in the dielectric composition bonded strongly by the sintering aid.
Further, since conventional dielectric compositions are fired at high temperatures of 1200xc2x0 C. or more, a problem of electrode break due to high temperature firing is caused even though a high dielectric constant is realized. Additionally, the size of crystal particles in the dielectric composition is increased, and thus there is a technical limitation to apply such a composition to a superthin type multi-layer ceramic capacitor.
Therefore, it is an object of the present invention to alleviate the problems in the prior art and to provide a low temperature firable dielectric composition, which is advantageous in light of increased homogeneity, dielectric constant and insulation resistance (specific resistivity).
It is another object of the present invention to provide a method of preparing such a dielectric composition.
It is a further object of the present invention a multi-layer ceramic capacitor using such a dielectric composition.